The invention relates to a method and an apparatus for controlling pick-off (doffing) and to a method of controlling the speed of flat item doffing apparatus such that a desired gap is maintained between doffed and to-be doffed items.
Doffing the frontmost shipments in each case from a stack of shipments must be done while maintaining a predetermined minimum gap. This minimum gap should not be undershot, so that downstream parts of the system are capable of handling the individual shipments. At the same time, the mean gap attained should exceed the minimum gap as little as possible, so that a high shipment throughput can be attained.
From European Patent Disclosure EP 0 167 091 A1, corresponding to German Patent Disclosure DE 34 24 397 A1, and German Patent DE 196 07 304 C1, apparatuses for doffing flat shipments from a stack are known. These apparatuses have a controlled doffing device, which doffs the frontmost shipment from a stack in each case and advances it to the grasping region of a pair of driven feed rollers; between the outlet of the stack and the feed rollers, there is a measuring segment in the form of a line of light gates, whose output signals are delivered to a control circuit. In EP 0 167 091 A1, the drive of the doffing device for doffing a shipment is effected such that the spacing between the shipment to be doffed and an already doffed shipment is ascertained, and the result of the spacing measurement in each case is corrected by a predetermined value dependent on the acceleration of the item to be doffed, and the doffing is triggered when the magnitude of the thus-corrected spacing measurement corresponds to a desired spacing value.
The use of the predetermined value makes it possible to take different mechanical properties of various shipments and their effect into account in the acceleration process.
A variable behavior of the shipments in the doffing operation also results from the variable position of the shipments in the stack from which the shipments are doffed. Once again, this leads to different gaps and hence to sacrifices in throughput.
For more-precise gap control, the shipment to be doffed is therefore, according to German Patent DE 196 07 304 C1, initially accelerated to an intermediate speed value, which is less a predetermined final speed. As soon as the actual spacing is equal to the desired spacing, the shipment is accelerated to the final speed.
These known embodiments require a complicated measuring segment in the form of a line of light gates, with which both the position of the doffed shipments (rear edge, as long as it is located inside the measuring segment) and the position of the next shipment to be doffed (front edge) are detected on an ongoing basis, in order to ascertain the correct times for accelerating the shipment to be doffed.
Furthermore, U.S. Pat. No. 4,541,624 describes a doffing apparatus in which the shipments, standing on the lower edge, are doffed upward; that is, the different lengths of shipments must be taken into account. Doffing and correction means are driven constantly, but at different fixed speeds, the respective duration of which is ascertained. No measure of the actual spacing of the shipment to be doffed from the doffed shipment is made. This spacing is not calculated until the shipment to be doffed has already been doffed; that is, defined shipment spacing cannot be assured.
The invention provides a method and an apparatus for picking off (doffing) flat shipments from a stack that, without a measuring segment of lined-up sensors for detecting the shipments, control the doffing in such a way that a minimum gap between the shipments that is optimal for further handling of the shipments is maintained.
There is no longer a need to measure the spacing between the doffed shipment from the shipment to be doffed on an ongoing basis in order to determine the time for accelerating the shipment to be doffed. Instead, the speed at which the shipment to be doffed, once it reaches the first sensor, must be transported to the second sensor in order to achieve the desired gap is ascertained; that is, a line of sensors is no longer necessary.